This invention relates to a method of controlling the exposure quantity in an automatic color picture printer in which density correction may be implemented without affecting the extent of the tri-color exposure quantity correction.
Conventional automatic color picture printers employ an integration neutralization system based on the fact that in ordinary photographic scenes the mean reflectivity of the three primary colors obtained by integrating the entire scene is substantially constant. Accordingly, a picture print having uniform density and color balance is obtained by bringing the integration value of the light transmission quantities of the three primary colors (blue, green and red) from the entire film negative close to a certain value. With such a control system, however, it is impossible to provide sufficiently satisfactory prints from a film negative having an extremely wide range of density and color balance. The main reasons for this are as follows:
(1) Irregularity of photographic papers (chemical factor)
(2) Delay in shutter operation (Mechanical factor)
(3) Irregularity of electrical circuits (Electrical factor)
(4) Imperfections in light receiving filters (Optical factor)
In order to eliminate the undesirable effects of these factors, a so-called slope circuit has been proposed, and is now employed in most conventional color printers. In such printers a reference or standard film negative is provided, and in printing another film negative the exposure quantity determined by the above-described method is corrected in accordance with the difference in density between the film negative being prepared and the reference film negative. More specifically, if it is assumed that the densities of the three colors of the reference film negative are represented by Dn, the exposure times of the three colors are represented by Tn, and the densities of the three colors of an optional film negative are represented respectively by Dy (yellow), Dm (magenta) and Dc (cyan), then the exposure times of the three colors for the new film negative can be determined by the following equations: ##EQU1## where Cy, Cm and Cc are constants representing the magnitudes of the slopes. If all of the constants are unity or one, there is no slope. If the density increases by one, the exposure time increases by a factor of ten.
In an exposure system where the densities of the three colors of a film negative are obtained and used to control the exposure times of the three colors, the following equations are employed instead of equations (1) above: ##EQU2##
According to these equations the original densities of the three colors in a film negative are converted into density values Dy', Dm' and Dc', and exposure control is then effected by utilizing these resultant density values as if they were the original densities of the film negative. Equations (2) above therefore determine the slope quantities.
When a film negative is color-balanced by the conversions defined by equations (2), no matter what density the film negative has, the integration of the densities of the three colors over the entire picture provides prints whose three colors are constant at all times. For instance, if the values Cy, Cm and Cc are satisfactorily controlled, prints which are well-balanced in color and density can be provided even if the densities (Dy, Dm, and Dc) of the three colors are, for example, (0.1, 0.1, 0.1) or (1.5, 1.5, 1.5). When prints are to be made from film negatives which are out of color balance, however, if no correction is applied the prints may appear correct in density but out of color balance. When the color of a print negative is magenta, for example, and the three color densities are (0, 1, 0), the exposure quantity is such that green light is increased but red and blue light are decreased. Color correction is therefore effected to some extent, and the resulting print appears rather gray. This correction is not sufficient, however, and therefore the print obtained is gray with dark green. The reason for this is as follows. As the color of the film negative is entirely magenta, the light incident on the green light sensor is rather dark. However, because of the imperfection of the green filter, light other than green light is applied thereto and the damping component of the actual green light is thus underdetected. On the other hand, the red and blue light sensor filters are also imperfect, and some green light is therefore detected by them. As a result, because of the added damping of the green light, the detection outputs erroneously indicate that the red light and blue light are correctly damped. Accordingly, in the respective light sensors color deviations are underdetected and color correction is under controlled. Thus, the resulting print is tinted with green rather than gray.
In addition, the light applied to photographic paper is not completely independent of coloring. That is, the light for producing one color may sometimes serve to reduce the other colors. Therefore, even if the Cy, Cm and Cc values are suitably adjusted to obtain prints from the film negative which are properly color balanced (that is, even if the density correction is perfect), in prints made from a film negative which is out of color balance the color correction will be imperfect. That is, the density correction is perfectly carried out, but at the expense of the color correction. This is a condition where the density correction is completely achieved, but the color correction is insufficient.
On the other hand, in order to obtain perfect color correction it is possible to adjust the values of Cy, Cm and Cc. That is, all that is necessary in this case is to adjust or control the Cy, Cm and Cc values in equations (2) so that no matter how much the film negative is out of color balance, the prints obtained are gray. It should be noted that the term "gray" here has a wide range, from a light gray to a dark gray. Regardless of the gray density, however, it is possible to control the values of Cy, Cm and Cc so that the resulting prints appear gray no matter how much the film negative is out of color balance. Such prints would appear dark gray with an "over" negative film, however, and light gray with an "under" negative film. This is a condition where the color correction is perfect, but the density correction is excessive.
As is apparent from the above description, the conventional slope circuitry as indicated by equations (1) and (2) is disadvantageous in that the color correction and density correction cannot both be perfect at the same time. This drawback is due to the fact that both color correction and density correction are effected simultaneously rather than independently, and each one may adversely affect the other. To overcome this drawback, a system in which color and density correction can be separately controlled should be employed, by dividing the system into a portion for controlling the extent of correction of the three colors and a portion for controlling the extent of the mean density correction.
In general, however, it is difficult to control the extent of correction of the three colors without affecting the density correction, because if it is intended to make a given color darker, the other colors are inherently made lighter owing to the principle of constant density control.